Method and structure for eliminating polarization instability in laterally—oxidized VCSELs

ABSTRACT

The polarization instability inherent in laterally-oxidized VCSELs may be mitigated by employing an appropriately-shaped device aperture, a misoriented substrate, one or more cavities or employing the shaped device aperture together with a misoriented substrate and/or cavities. The laterally-oxidized VCSELs are able to operate in a single polarization mode throughout the entire light output power versus intensity curve. Combining the use of misoriented substrates with a device design that has an asymmetric aperture that reinforces the polarization mode favored by the substrate further improves polarization selectivity. Other device designs, however, can also be combined with substrate misorientation to strengthen polarization selectivity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/389,160filed Sep. 2, 1999 now U.S. Pat. No. 6,304,588, which is a Continuationin Part claiming the benefit of application Ser. No. 09/364,614 filedJul. 29, 1999 now abandoned which is a Continuation in Part ofapplication Ser. No. 08/940,867 filed Sep. 30, 1997 now U.S. Pat. No.5,978,408, which claims the benefit of Provisional Application60/037,175 filed Feb. 7, 1997.

FIELD OF INVENTION

The present invention relates generally to semiconductor lasers. Morespecifically, the invention allows for the elimination of thepolarization instability in laterally-oxidized vertical-cavity surfaceemitting lasers.

BACKGROUND OF INVENTION

Solid state semiconductor lasers are important devices in applicationssuch as optoelectronic communication systems and high-speed printingsystems. Recently, there has been an increased interest in verticalcavity surface emitting lasers (“VCSEL's”) although edge emitting lasersare currently used in the vast majority of applications. A reason forthe interest in VCSEL's is that edge emitting lasers produce a beam witha large angular divergence, making efficient collection of the emittedbeam more difficult. Furthermore, edge emitting lasers cannot be testeduntil the wafer is cleaved into individual devices, the edges of whichform the mirror facets of each device. On the other hand, not only doesthe beam of a VCSEL have a small angular divergence, a VCSEL emits lightnormal to the surface of the wafer. In addition, since VCSEL'sincorporate the mirrors monolithically in their design, they allow foron-wafer testing and the fabrication of one-dimensional ortwo-dimensional laser arrays.

A known technique to fabricate VCSEL's is by a lateral oxidationprocess, as schematically illustrated in FIGS. 1 and 2. Under thisapproach, a laser structure comprising a plurality of layers is formedupon substrate 10. These layers include an active layer 12 and an AlGaAslayer 14 with a high aluminum content. The AlGaAs layer 14 is placedeither above or below the active layer of a laser structure. Then, thelayered structure is masked and selectively etched to form a mesastructure 22 as illustrated in FIG. 2. As a result of the etching, theAlGaAs layer 14 with a high aluminum content adjacent to the activelayer 12 is exposed at the edges of the mesa structure 22. To form thelasing emissive region or “aperture”, this AlGaAs layer is oxidizedlaterally from the edges towards the center of the mesa structure asrepresented by arrows A. Other layers in the structure remainessentially unoxidized since their aluminum content is lower.Consequently, their oxidation rates are also substantially lower.Therefore, only the AlGaAs layer with high aluminum content is beingoxidized. The oxidized portions of the high aluminum content layerbecome electrically non-conductive as a result of the oxidation process.The remaining unoxidized region, which is conductive, in the AlGaAslayer forms the so-called “aperture”, a region which determines thecurrent path in the laser structure, and thereby determines the regionof laser emission. A VCSEL formed by such a technique is discussed in“Selectively Oxidized Vertical Cavity Surface Emitting Lasers With 50%Power Conversion Efficiency,” Electronics Letters, vol. 31, pp. 208–209(1995).

The current lateral oxidation approach has several disadvantages, suchas large mesa, large oxidation region, and poor control of the aperturesize. A key disadvantage of this approach is the difficulty incontrolling the amount of oxidation. Generally, the desired deviceaperture is on the order of one to ten microns (μm), which means thatseveral tens of microns of lateral oxidation will typically be requiredin order to fabricate the device when oxidizing in from the sides of themuch larger mesa, which must typically be 50 to 100 microns in size.Since the size of the resulting aperture is small relative to the extentof the lateral oxidation regions, the devices formed generally havesevere variations in aperture size as a result of non-uniform oxidationrates from wafer to wafer and across a particular wafer. The oxidationrate of AlGaAs depends strongly on its aluminum composition. Anycomposition non-uniformity will be reflected by changes in the oxidationrate, which in turn creates uncertainty in the amount of oxidation. Theprocess is also relatively temperature-sensitive. As the oxidation ratevaries, it is difficult to ascertain the extent to which a laserstructure will be oxidized, thereby decreasing reproducibility in deviceperformance. In short, such a process often creates variousmanufacturability and yield problems.

Another disadvantage of a VCSEL formed by a traditional lateraloxidation approach is the difficulty it creates in forming high densitylaser arrays. In order to oxidize a buried layer of high aluminumcontent, an etching process is performed leaving a mesa. After theetching of this mesa, lateral oxidation is performed such that theoxidized regions define a laser aperture of a particular size. The useof a mesa structure, in part, limits the minimum spacing between twolasers in an array. The step height of the mesa is typically severalmicrons because of the need to etch through a thick upper DBR mirror.Additionally, the top surface of the mesa also has to be relativelylarge so that a metal contact can be formed on it without covering thelasing aperture. Typically, the minimum size of an electrical contact isapproximately 50×50 μm². Hence, the step height of the mesa and theplacement of the electrical contact on the surface make it difficult toform highly compact or high density laser arrays.

A solution to some of the problems associated with a typical mesastructure is the use of a shallow mesa. In order to use a shallow mesa,the upper mirror is not formed by an epitaxial process. Instead, theupper mirror is formed by a deposited multilayer dielectric material,which reflects light. Electrical contact is made directly onto the upperportion of the active region. Devices formed under this approach havebeen fabricated on mesas with widths of approximately twelve microns.However, the added complexity of depositing a dielectric material andusing a liftoff process to define the contact make it difficult tooptimize the devices for low threshold current and high efficiency.

A VCSEL formed by a traditional lateral oxidation approach often suffersfrom poor mechanical or structural integrity. It is well-known that theupward pressure applied during a packaging process may causedelamination of the entire mesa since the bonding of the oxide layer tothe unoxidized GaAs or AlGaAs is generally weak.

Light from typical VCSELs is usually polarized along one of twoorthogonal directions along the wafer surface. The dominant polarizationcan switch back and forth between these two orthogonal orientations asthe operating current to the VCSEL is varied because there is no naturalpreference for either orthogonal direction. The polarization instabilityis a major drawback because it limits VCSELs to applications where nopolarization sensitive optical elements are present. Moreover, if theVCSEL is modulated, sudden changes in polarization states can result inundesirable light intensity fluctuations that contribute to signalnoise.

There are several known methods for controlling VCSEL polarization.These include making devices with anisotropic mesa geometries asdescribed by K. Choquette and R. Leibenguth in “Control ofvertical-cavity laser polarization with anisotropic cavity geometries”,IEEE Photonics Technology Letters, vol. 6, no. 1, pp. 40–42, 1994,making devices with tilted etched-pillar structures as described by H.Y. Chu et al. in “Polarization characteristics of index-guided surfaceemitting lasers with tilted pillar structure”, IEEE Photonics TechnologyLetters, vol. 9, no. 8, pp. 1066–1068, 1997, use of dielectric topmirrors with coated sidewalls as described by M. Shimuzi et al. in“Polarisation control for surface emitting lasers”, Electronics Letters,vol. 27, no. 12, pp. 1067–1069, 1991, using substrates having amisoriented surface as described in Compound Semiconductor, May/June, p.18, 1997 or milling a cavity next to a completed gain-guided device asdescribed by P. Dowd et al. in “Complete polarisation control of GaAsgain-guided top-surface emitting vertical cavity lasers”, ElectronicLetters, vol. 33, no. 15, pp. 1315–1317, 1997.

BRIEF SUMMARY OF INVENTION

Large arrays of densely-packed VCSELs are attractive light sources forapplications such as laser printbars, where there may be thousands ofsemiconductor lasers on a small chip operating to transfer print imagesat high speed. Laterally-oxidized VCSELs are of particular interestbecause these VCSELs operate with exceedingly low threshold currents andhigh efficiencies, properties that are important for densely-packedVCSEL arrays. The polarization instability inherent inlaterally-oxidized VCSELs may be mitigated by employing anappropriately-shaped device aperture, a misoriented substrate, one ormore cavities or employing the shaped device aperture together with amisoriented substrate and/or cavities. The laterally-oxidized VCSELs areable to operate in a single polarization mode throughout the entirelight output power versus intensity curve.

While a certain degree of polarization selectivity can be achieved bymaking devices on substrates whose surfaces are misoriented from, forexample, the {100} surface, this method often does not producesufficient polarization selectivity. A more effective solution involvescombining the use of misoriented substrates with a device design thathas an asymmetric aperture that reinforces the polarization mode favoredby the substrate. Other device designs, however, can also be combinedwith substrate misorientation to strengthen polarization selectivity.

An alternative device design is discussed in “Complete polarisationcontrol of GaAs gain-guided top-surface emitting vertical cavity laser”by P. Dowd, et al. In this VCSEL, deep 1 μm wide cavities placed between1 and 2 μm from the cavity aperture produce differential loss for thetwo polarization modes. The cavities are formed after device fabricationusing a focus ion beam etcher. The favored polarization mode using thismethod is found to be in a direction perpendicular to the cavity. Inthis example, enhanced polarization selectivity can by achieved byfabricating the VCSEL on a misoriented substrate and aligning the cavityalong a direction perpendicular to the polarization mode favored by themisoriented substrate.

Another method of producing polarization selectivity involves applyingan anisotropic stress either by external means or by a built inmechanism as discussed in “Engineered polarization control ofGaAs/AlGaAs surface-emitting lasers by anisotropic stress fromelliptical etched substrate hole” by T. Mukaihara, et al. An ellipticalhole is first etched and a high thermal expansion coefficient materialis then deposited on the hole. The high thermal expansion material canbe a thin film, an epitaxial layer, or an adhesive. The resultinganisotropic stress produces a gain difference between the twopolarization modes resulting in a favored polarization direction alongthe short axis of the elliptical hole. Enhanced polarization selectivitycan again be achieved by fabricating the VCSEL on a misorientedsubstrate and aligning the short axis of the elliptical hole to thepolarization direction favored by the misoriented substrate.

A VCSEL employing an asymmetric etched mesa can also producepolarization preference as illustrated in “Control of vertical-cavitylaser polarization with anisotropic transverse cavity geometries” by K.Choquette, et al. VCSELs utilizing dumbbell-shaped mesas havepolarization preferences along the long axis of the dumbbell. Thepolarization preference can again be strengthened by making the deviceon a misoriented substrate and by positioning the long axis of thedumbbell-shaped mesa along a polarization direction favored by themisoriented substrate.

There are various means of producing a polarization preference bydifferent VCSEL designs. However, most VCSEL designs do not producesufficient polarization selectivity to completely suppress the nonpreferred polarization mode. The polarization selectivity can besignificantly improved by fabricating these devices on misorientedsubstrates and designing the VCSELs so that their favored polarizationdirection reinforces the polarization preference that is inherent in themisoriented substrate.

The advantages and objects of the present invention will become apparentto those skilled in the art from the following detailed description ofthe invention, its preferred embodiments, the accompanying drawings, andthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate a prior art approach to the formation of alaser aperture in a VCSEL structure.

FIG. 3 illustrates a side sectional view of a semiconductor structurewhich is used to form the preferred embodiment of the present invention.

FIG. 4 is a top view of a portion of a mask which may be applied to thesemiconductor structure shown in FIG. 1 under the present invention.

FIG. 5 is a partial side sectional view of the semiconductor structureof FIG. 1 with a cavity etched therein.

FIG. 6 is a simplified top view of a portion of an oxidation layer,wherein the layers above it have been removed.

FIG. 7 is a cross-sectional view taken substantially along line 7—7 inFIG. 6 and in FIG. 9.

FIG. 8 is a cross-sectional view taken substantially along line 8—8 inFIG. 6 and in FIG. 9.

FIG. 9 is a top view of two adjacent VCSEL structures showing anon-transparent top contact.

FIG. 10 is a top view of two adjacent VCSEL structures showing atransparent top contact.

FIG. 11 shows a laser structure whose aperture is defined by atriangular bounding pattern of cavities.

FIG. 12 shows an array of lasers which is formed by repeating thetriangular bounding pattern shown in FIG. 11.

FIG. 13 shows a laser structure whose aperture is defined by a boundingpattern of four cavities arranged in a square pattern.

FIG. 14 shows an array of lasers which is formed by repeating the squarebounding pattern shown in FIG. 13.

FIG. 15 shows another array of lasers which is formed by repeating thesquare bounding pattern shown in FIG. 13.

FIG. 16 shows a laser structure whose aperture is defined by a boundingpattern of six cavities arranged in an hexagonal pattern.

FIG. 17 shows an array of lasers which is formed by repeating thehexagonal bounding pattern shown in FIG. 16.

FIG. 18 shows another array of lasers which is formed by an alternativerepeating of the hexagonal bounding pattern shown in FIG. 16.

FIG. 19 shows a typical planar laterally oxidized VCSEL.

FIG. 20 shows a light output power versus current plot for a typicalplanar laterally oxidized VCSEL.

FIG. 21 shows an embodiment of a planar laterally oxidized VCSEL inaccordance with the invention.

FIG. 22 shows a light output power versus current plot for theembodiment shown in FIG. 21.

FIG. 23 shows a misoriented substrate relative to standardcrystallographic orientations.

FIG. 24 shows the orientation of the electric field vector relative tomisoriented substrate.

FIG. 25 shows gain anisotropies for a misoriented substrate.

FIG. 26 shows an embodiment of a planar laterally oxidized VCSEL inaccordance with the invention.

FIG. 27 shows an embodiment of a planar laterally oxidized VCSEL inaccordance with the invention.

DETAILED DESCRIPTION

FIG. 3 illustrates a semiconductor structure which is used to form thepreferred embodiment of the present invention. The structure illustratedincludes a number of semiconductor layers, which can be used to form avertical cavity surface emitting laser. As will be apparent, the layersare illustrated schematically only and bear no relationship to therelative thicknesses each to the other. As shown in FIG. 3, an n-typeGaAs buffer layer 102 of approximately 200 nanometers is grown on ann-type GaAs substrate 100 using an epitaxial deposition process known asmetal-organic chemical vapor deposition (“MOCVD”). The doping level ofthe n-type GaAs substrate and GaAs buffer are typically around the rangeof 3×10¹⁸ cm⁻³ to 7×10¹⁸ cm⁻³ so that a reasonably low resistance can beachieved in these layers. The semiconductor layers may also be depositedon a substrate by liquid phase epitaxy (“LPE”), molecular beam epitaxy(“MBE”), or other known crystal growth processes.

Above the GaAs buffer layer 102 is a superlattice structure for forminga lower distributed Bragg reflector (“DBR”) 104 which provides a portionof the necessary internal reflection in a VCSEL structure. The lower DBR104 is typically formed by multiple pairs of an AlGaAs layer with a highaluminum content (approximately 86% aluminum) and another AlGaAs layerwith a low aluminum content (approximately 16% aluminum). After thegrowth of a number of layer pairs (typically 35 Si dopedpseudoparabolically graded DBR pairs), a final AlGaAs layer with a highaluminum content is deposited before growing the first cladding layer106 of the optical cavity. A typical thickness of each layer pair isapproximately 120 nanometers for a laser emitting at 820 nanometers. Thetotal thickness of each layer pair is designed to be equal to one halfof the optical wavelength at the intended wavelength of laser operation.The thickness of the final high aluminum content layer is designed to bea quarter of the optical wavelength at the intended wavelength of laseroperation. The AlGaAs layer with a high aluminum content containsapproximately 86% aluminum. The aluminum content of the AlGaAs layerwith a high aluminum content should be sufficiently high to provide fora low refractive index, but not so high as to oxidize easily. The AlGaAslayer with a low aluminum content has an aluminum content ofapproximately 16%. The composition of the AlGaAs layer with a lowaluminum content should typically have sufficient aluminum so that it isnon-absorptive at the lasing wavelength.

Under this embodiment, since light is outcoupled through the top surfaceof the semiconductor sample, the reflectivity of the lower DBR 104should be as close to 100% as possible in order to achieve high internalreflection. High internal reflection generally reduces the thresholdcurrent of a laser. It is well-known that the reflectivity of the lowerDBR 104 is generally a function of the difference in the refractiveindices between the two AlGaAs layers of the superlattice and the numberof layer pairs in the structure. The greater the difference in therefractive indices, the fewer number of pairs are required to obtain agiven reflectivity. For example, 30 to 40 pairs of AlGaAs layers may beused to form the lower DBR structure 104.

After the lower DBR structure 104 has been deposited epitaxially, anAlGaAs cladding layer 106 is deposited. This lower AlGaAs cladding layer106 has an aluminum content of about 58% and is n-type with a dopinglevel of 1×10¹⁸ cm⁻³ to 5×10¹⁸ cm⁻³. Its thickness is approximately 100nanometers. Above this AlGaAs cladding layer 106 is the active layer 108of the laser structure which comprises four InAlGaAs quantum wells witha thickness of about four to ten nanometers, typically about fournanometers, along with five Al_(0.35)Ga_(0.65)As barriers with athickness of about two to eight nanometers, typically about sixnanometers. Depending upon the desired output wavelength of the laserstructure, pure GaAs or AlGaAs with a low aluminum content may be alsoused to form the quantum well structures. Nothing in this inventionprevents the use of a single quantum well or other multiple quantum well(“MQW”) structures to form the active layer 108.

Above the active layer 108 is an upper AlGaAs cladding layer 110, whichis structurally similar to the lower AlGaAs cladding layer 106 exceptfor the polarity of its dopants. This upper cladding layer 110 has analuminum content of about 58% but is p-type with a doping level of1×10¹⁸ cm⁻³ to 4×10¹⁸ cm⁻³. Similar to the lower AlGaAs cladding layer106, the thickness of top cladding layer 110 is also about 100nanometers. These two AlGaAs cladding layers, 106 and 110, along withthe active layer 108 generally form the optical cavity in which thedesired optical gain can be attained. The total optical thickness oflayers 106, 108, and 110 is adjusted to be equal to an integer multipleof the intended wavelength of laser operation.

Above the upper AlGaAs cladding layer 110 is an oxidation layer 112,which is used to form the laser aperture. The laser aperture controlsthe current flow and thus the lasing location in the active layer 108.Under this embodiment, this oxidation layer 112 is above the upperAlGaAs cladding layer 110. Nothing in this invention prevents theplacement of this oxidation layer 112 in another location either furtherabove or below the active layer 108. Typically, this oxidation layer 112has an aluminum content of approximately 95% and a thickness of about 70nanometers. Typically, this oxidation layer 112 constitutes the firstlayer of an upper DBR mirror and contains a p-type dopant.

After the oxidation layer 112 has been formed, the remainder of an upperDBR mirror 114 which contains p-type doping is deposited. The upper DBRmirror 114 is structurally similar to the lower DBR mirror 104 exceptfor the polarity of its dopants. Additionally, the mirror layer closestto each side of the active region generally has a high aluminum content.In this embodiment, this high aluminum content layer is also theoxidation layer 112. In this embodiment, the reflectivity of the upperDBR 114 is typically 98% to 99% because light will be outcoupled throughthe surface of the semiconductor sample. Typically, 20 to 25 pairs ofalternate AlGaAs layers are used to form this upper DBR mirror 114.Typically, a p-AlGaAs current spreading layer and a final 22 nanometerthick p⁺ GaAs layer are grown above top DBR mirror 114.

FIG. 4 is a top view of a portion of a mask which may be applied to thesemiconductor structure shown in FIG. 3 under the present invention.First, as is conventional, a uniform layer of silicon nitride will bedeposited over the entire semiconductor sample. Then, a photoresistlayer 118 is deposited over the silicon nitride layer and is formed intothe mask shown in FIG. 4 by a photolithographic process which removesphotoresist material from four circular areas 120. The circular areas120 form a pre-determined bounding pattern which will later be used todefine the resulting aperture of a laser structure.

As illustrated in FIG. 5, the sample then undergoes an etching processduring which cylindrical cavities 126 are formed in the semiconductorstructure through the four exposed circular areas 120. The etching isperformed by a process such as reactive ion etching which provides forthe formation of a deep depression with vertical sidewalls. The depth ofeach cylindrical cavity should extend at least into the oxidation layer112, as shown in FIG. 5. After the formation of the cylindrical cavitiesand the removal of any photoresist on the surface, the semiconductorsample undergoes an oxidation. The sample is typically oxidized withwater vapor in a nitrogen environment at elevated temperatures, inexcess of 350° C. During the oxidation process, the oxidation layer 112is exposed to the ambient through each cylindrical cavity, as indicatedby arrows A. Thus, the oxidation layer 112, which comprises of AlGaAswith a high aluminum content, is oxidized radially outwards from eachcavity 126, typically until the oxidized regions 124 surrounding eachcavity approach one another and overlap, as can be seen in FIG. 6.However, a small non-oxidized gap between the oxidized regions may bepermissible so long as electrical and optical fields are adequatelyconfined. Although the cross section of each cavity has been describedas being cylindrical, any suitable cross section may be used.

During the oxidation process, other layers in the structure remainessentially unoxidized since their aluminum content is lower. Theoxidation rate of AlGaAs increases with the aluminum content in agenerally exponential manner at constant temperature. The time durationof the oxidation process depends upon the aluminum content in theoxidation layer 112 and the oxidation temperature. A desirable,controllable oxidation duration would be a few tens of minutes.Therefore, the layer that is being oxidized is the AlGaAs which has ahigh aluminum content of close to 95%. The portion of the AlGaAs layerwhich remains unoxidized controls the current path through the activelayer 108.

FIG. 6 is a largely simplified top view of the oxidation layer 112 shownin FIG. 3 assuming that all the layers above it have been removed. Theshaded region 122 represents the laser aperture in oxidation layer 112which determines the region of laser emission by active layer 108. It isformed by the oxidation process of the present invention. During theoxidation process, the oxidation fronts emanate through the oxidationlayer from the pattern of four cavities 126, and the shaded region 122is formed by the intersection of the boundaries of the oxidized regions124. The oxidation fronts emanating from the cylindrical cavities 126are also generally cylindrical, resulting in overlapping oxidizedregions 124. The center of the overlapping regions 124 remainsunoxidized. This unoxidized region forms the shaded area 122, which isthe aperture of the laser structure. After the oxidation process, an ionimplantation process, which is next described, is used to form isolationregion 130 to isolate the laser structure from its neighbor.

After the oxidation process, the areas 124 are oxidized and theunoxidized portion 122 forms the aperture which controls the currentpath through the active layer 108. Current flow through that portion ofthe active layer 108 which lies below the aperture 122 results in aninjected density of p-type and n-type carriers, resulting in opticalamplification. At sufficiently high current flow, this opticalamplification, in combination with feedback from the DBR mirrors, 104and 114, will result in laser oscillation and emission from the activelayer within the region defined by aperture 122 in oxidation layer 112.

Isolation region 130 (illustrated in FIGS. 6, 7 and 8), which is formedby using an ion implantation isolation process, is highly resistive. Thetypical implantation energies used in such a process are 50 KeV, 100KeV, 200 KeV and 310 KeV. The dose is typically 3×10¹⁵ cm⁻² at eachenergy level. The ion used to form the isolation region 402 is typicallyhydrogen.

After the isolation process, metal contacts 132 and 134 are formed onthe top surface and the bottom surface of the semiconductor structure,respectively, for biasing the laser, as illustrated in FIGS. 7, 8 and 9.A typical material used for forming the contacts is a titanium/goldbilayer film.

FIG. 9 shows a top view of a VCSEL structure formed in accordance to thepresent invention after a typical metallization process to form the topcontact 132. Views in the direction of section lines 7—7 and 8—8 in thisFigure are also as illustrated in FIGS. 7 and 8. The top contact 132 isof a generally keyhole shape, including a circular portion 134 and anextension portion 136. The circular portion lies inboard of the cavities126 and overlies the laser aperture 122. Since it is non-transparent itis made annular in shape so as to allow light to be coupled out of thelaser through its central opening. The width “W” of the annular circularportion 134 is usually limited by the minimum line width achievableunder the processing technology used, thus setting a lower limit on thepitch between adjacent VCSEL structures Thus, a typical pitch betweenthe centers of two adjacent VCSEL apertures 122 would be “4W.” However,if a transparent conductor is used (e.g. see FIG. 10), the pitch betweenadjacent VCSEL structures could be further reduced to be on the order of“2W” because the top contact could overlie the laser aperture 122.Moreover, the contact arrangement provides a direct current path to theoptical mode at the center of aperture 122 and may be useful inapplications where reduced mode partition noise is desired.

A typical transparent conductor is indium tin oxide (“ITO”) which isdeposited by a sputtering process prior to etching cavities 126 asdescribed above. This procedure is self-aligned and greatly simplifiesfabrication and is enabled by the stability of ITO during the lateraloxidation process (see “Low-threshold InAlGaAs vertical-cavitysurface-emitting laser arrays using transparent electrodes” by C. L.Chua et al. in Applied Physics Letters, vol. 72, no. 9, 1001, which isincorporated by reference in its entirety). A half-wavelength thick ITOfilm is first deposited over the p⁺ GaAs contact layer overlyingp-AlGaAs current spreading layer which is grown over DBR layer 114. TheITO film is then successively rapid thermal annealed at 300 and at 600°C. for 2 min each in a nitrogen ambient. The low-temperature annealcrystallizes the deposited amorphous ITO film, while the second,higher-temperature anneal induces ohmic contact formation between theITO film and the p⁺ GaAs contact layer. The transparent ITO film attainsa post anneal contact resistance of 2×10⁻⁵ ohm cm², a sheet resistivityof 5×10⁻⁴ ohm cm, and a power transmission coefficient of 98% at anemission wavelength of 817 nanometers.

Next a set of cavities 126, typically having a diameter of 2 μmdelineating laser aperture 122 is patterned as shown in FIG. 10 forexample. The ITO and underlying DBR layers 114 are then dry etched usingchemically assisted ion beam etching in two successive self-alignedsteps to form cavities 126 that reach oxidation layer 112, typicallyAl_(0.94)Ga_(0.06)As. Oxidation layer 112 is oxidized for 45 minutes at415° C. in flowing steam. Oxidized regions 124 progress laterallyoutwards from each cavity 126, and on merging define laser aperture 122.Apertures 122 may typically range from 5 μm to 43 μm in diameter.

Positioning of cavities 126 is typically selected so that lateraloxidation needs to proceed by only a few micrometers from the perimetersof cavities 126. This reduced path of oxidation compared to typicaletched pillar devices reduces the sensitivity of laser aperture 126 tovariations in oxidation rates. The aluminum content of oxidation layer112 is relatively low in order to lengthen the oxidation time so thattransients are minimized. As noted above, the ITO film is not effectedby the oxidation process.

An alternative embodiment of the top contact is shown in FIG. 10 and isidentified by numeral 138. It comprises a transparent conductive contactfinger 140 and contact pad 142, the contact finger overlying the laseraperture 122. After the formation of an electrical contact on the topsurface, the bottom electrode 134 is formed by depositing metal on thebottom surface of the substrate 100 and is typically an evaporatedeutectic Ge/Au metal.

FIGS. 11, and 12, 13, 14 and 15, and 16, 17 and 18 illustratealternative packing arrangements for forming an array of lasers formedby the method of the present invention. In the laser device of FIG. 11and an array of such devices shown in FIG. 12, each laser structureincludes an aperture 222 defined by oxidized regions 224 surrounding abounding pattern of three cylindrical cavities 226 positioned at theapexes of an equilateral triangle. The spacing between the centers ofany two cavities is “S.” As stated previously, during the oxidationprocess, an embedded AlGaAs layer with a high aluminum content will beoxidized radially outwardly from the cylindrical cavities 226 until theoxidized regions 224 overlap to form the unoxidized laser aperture 222.The packing arrangement shown in FIG. 11 may be repeated to form a laserarray as shown in FIG. 12. If the spacing between the centers of twocylindrical cavities is “S,” a typical linear spacing “L” between twolaser apertures is approximately “S/2.”

In the laser device of FIG. 13 and the arrays of FIGS. 14 and 15, thesquare bounding pattern of cylindrical cavities 126 is illustrated.Oxidized regions 124 will overlap to form the unoxidized laser aperture122. This packing arrangement shown in FIG. 13 may be repeated to form alaser array as shown in FIG. 14 or 15 If a packing arrangement such asFIG. 14 is used and the spacing between the centers of two adjacentcylindrical cavities is “S,” a typical linear spacing “L” between twolaser apertures is approximately “S.” If an arrangement such as FIG. 15is used and the spacing between the centers of two cylindricaldepressions is “S,” a typical linear spacing “L” between two laserapertures is approximately “√2×S.”

In the laser device of FIG. 16 and the arrays of FIGS. 17 and 18 anhexagonal bounding pattern of cylindrical cavities is illustrated. Itshould be apparent that the cavities 326 may also be arranged at thevertices of any other polygon. As in the previously describedembodiments, the laser aperture is formed by the unoxidized region 322defined by the oxidized regions 324. The packing arrangement shown inFIG. 16 may be repeated to form a laser array as shown in FIG. 17 or 18.If an arrangement such as FIG. 17 is used and the spacing between thecenters of two adjacent cylindrical cavities is “S,” a typical linearspacing “L” between two laser apertures is approximately “1.5S.” If anarrangement such as FIG. 18 is used, the closest linear spacing “L”between two laser apertures is approximately “√3×0.5S.”

The composition, dopants, doping levels, and dimensions given above areexemplary only, and variations in these parameters are permissible.Additionally, other layers in addition to the ones shown in the figuresmay also be included. Variations in experimental conditions such astemperature and time are also permitted. Lastly, instead of GaAs andGaAlAs, other semiconductor materials such as GaAlSb, InAlGaP, or otherIII-V alloys may also be used.

The planar laterally-oxidized (PLO) VCSELs described above utilize holesor cavities 126, 226 or 326 to penetrate upper DBR mirror 114. Cavities126, 226 or 326 serve to expose buried high aluminum layer 112 that isthen selectively oxidized. Cavities 126, 226 or 326 may be arranged atthe vertices of a polygon such that upon oxidation, the oxidizedregions, such as oxidized regions 124 of cavities 126 border VCSELaperture 122. Because oxidized regions 124 bordering aperture 122 have arefractive index lower than the refractive index of aperture 122 and areelectrically insulating, oxidized regions 124 form a good lateralwaveguide that also functions to confine current to aperture 122. Theplanar surface areas between cavities 126 allows electrical contactingand routing to be established in a planar manner. Inter-device isolationis accomplished using ion implantation.

FIG. 19 shows planar laterally oxidized (PLO) VCSEL 400. Typically,cavities 426 have a 2 μm diameter and cavities 426 are placed at thevertices of a regular octagon. Cavities 426 are typically positionedwith a center to center spacing of about 5 μm. Oxidation regions 424extend by about 3.5 μm from the edges of cavities 126, typically leavingaperture 422 with a 4 μm width. Light is emitted from aperture 422 ofVCSEL 400 through ITO electrode 438.

FIG. 20 shows the light output power versus current characteristics ofan embodiment of VCSEL 400. Curve 2071 shows the light from VCSEL 400with no polarization filter applied. Curve 2072 shows the light fromVCSEL 400 having a polarization along direction 2320 (see FIG. 23)inclined at an angle of about 4.1 degrees relative to the [011]crystallographic direction. Curve 2073 shows light from VCSEL 400 havinga polarization along the [011] crystallographic direction. FIG. 20 showsthat the light from VCSEL 400 is initially polarized along direction2325 which is the [011] direction but that the polarization switchesabruptly to direction 2320 (see FIG. 23) when the current reachesapproximately 0.8 mA as is indicated by the sudden drop in curve 2073and the corresponding rise in curve 2072. Curve 2073 rises again atcurrents above about 1.25 mA indicating the reappearance of a [011]polarized lasing mode. However, curve 2072 remains greater than curve2073 between about 1.25 mA and 2.25 mA, which shows that the dominantpolarization mode is along direction 2320 in this current range. Thedominant polarization mode switches to direction 2325 beyond about 2.25mA.

Embodiments of VCSEL 400 that are seemingly identical may behavedifferently with respect to the polarization direction and polarizationswitching as shown in “Anisotropic apertures for polarization-stablelaterally oxidized vertical-cavity lasers” by Chua et al., AppliedPhysics Letters vol. 73, no. 12, pp. 1631–1633 which is incorporated byreference in its entirety. This is indicative of the polarizationinstability inherent in conventional devices such as, for example, VCSEL400.

A stable polarization can be achieved if the symmetry between twoorthogonal axes is broken by a sufficiently large perturbation. In anembodiment in accordance with the invention, FIG. 21 shows this symmetrybreaking may be created by making aperture 522 asymmetric by arrangingholes or cavities 426 at the vertices of a distorted octagon. Thedistorted octagon is compressed by, for example, about 1.5 μm along the[011] direction and elongated by 1.5 μm along direction 2320 (see FIG.23). Upon oxidation, oval-like aperture 522 is formed. Ion implantationis performed outside of ellipsoidal region 531 consistent with thediscussion above.

During oxidation, AlGaAs layer 124 surrounding aperture 122 contractsand the change in thickness of layer 124 results in mechanical stressesat the boundary between layer 124 and aperture 122 (see FIG. 7). Theanisotropic stress resulting from oval-like aperture 522 removes thepolarization modal gain degeneracy. Hence, a significant polarizationpreference is established along one of the two orthogonal axes resultingin stable polarization independent of the current level in the operatingrange. The difference in gain available to the two orthogonalpolarization states is due to the differential gain that develops withthe asymmetric stress and the different modal gain resulting fromstress-induced birefringence.

FIG. 22 shows polarization-resolved light output power versus currentcurves obtained from an embodiment of VCSEL 500 in accordance with thepresent invention. Curve 2271 shows the light output power withoutpolarization filter. Curve 2272 shows that laser output in direction2320 (see FIG. 23) is completely suppressed throughout the operatingregime. Curve 2273 shows that VCSEL 500 displays stable polarizationalong the [011] direction throughout the operating regime. Thepolarization suppression ratio is 18 dB for curve 2272 relative to curve2273 at a current level of about 2.5 mA where peak light output power isreached.

Asymmetric apertures 522 on VCSELs 500 that are rotated ±90° from theorientation shown in FIG. 25 exhibit an enhanced output withpolarization in direction 2320 relative to symmetric aperture 422 butlaser light polarized in the [011] direction is not completelysuppressed if the substrate orientation favors the [011] polarizationdirection. Suppression of laser light polarized in all but the desiredpolarization direction is possible if both the substrate orientation andthe aperture asymmetry favor laser light polarized in the desiredpolarization direction.

Substrate 2300 (see FIG. 23) used in one embodiment of VCSEL 500 inaccordance with the present invention has the characteristics as shownin FIG. 22 with a substrate surface cut in a crystal plane that istilted toward the [011] crystallographic axis. FIG. 23 shows misorientedsubstrate 2300 with surface vector 2310 misoriented relative to [100]direction 2305 of a (100) oriented substrate surface. The misorientationis by angle of rotation β about [011] direction 2325 toward [011]direction 2315. Misorientation relative to any of the <111> axes alsoresults in anisotropic polarization selectivity. The <111> axes areoriented at an angle θ relative to the <100> axes where sin² θ=⅔. Forthe embodiment of FIG. 22, angle of rotation β is about 4.1 degrees.Groups of VCSEL 500 may be made in arrays resulting, for example, inarrays similar to those shown in FIGS. 17 and 18 with printer and otherapplications.

FIG. 24 shows the orientation of electric field vector E at polarizationangle α′ with respect to axis 2320 of an embodiment of misorientedsubstrate 2300 in accordance with this invention for light exitingsubstrate 2300. FIG. 25 shows the corresponding gain that is achieved inarbitrary units versus the polarization angle α′ for an embodiment ofmisoriented substrate 2300. The gain is seen to vary with polarizationangle α′ in a periodic manner. The gain is higher for E fields polarizedalong ±[011] direction 2325. Therefore, misoriented substrate 2300favors laser light polarized in ±[011] direction 2325 over laser lightpolarized in ±direction 2320.

However, substrates that are misoriented along a different direction andby different angles can also produce gain anisotropies. Since standard(100)-oriented substrates have crystal symmetries that belong to theD_(4th) point group, their gain properties are isotropic in thesubstrate plane as a function of angle. Misoriented substrates, however,can have symmetries that produce gain anisotropies leading todirectional gain dependencies as shown, for example, in FIG. 25 for anembodiment of substrate 2300.

Gain curves for a given substrate orientation can be determined by firstcalculating the quantum wave functions using the multiband effectivemass theory for the valence band and Kane's model (e.g., see E. O. Kane,in Journal of Physics and Chemistry of Solids, v. 1, p. 249, (1957),incorporated by reference in its entirety) for the conduction band. Inthe multiband effective mass theory, the valence band Hamiltonian for a(100) substrate consists of the Luttinger-Kohn Hamiltonian (e.g., see J.M. Luttinger and W. Kohn in Physical Review, v. 97, p. 869 (1955),incorporated by reference in its entirety) and a strain-orbit potentialterm if the active layer is under stress. Details regarding thestrain-orbital term may be found in G. E. Pikus and G. L. Bir in SovietPhysics-Solid State, vol. 1, 1502 (1960) incorporated by reference inits entirety.

Several sources of stress exist. First, stress on active layer 108 (seeFIG. 3) occurs because of the lattice mismatch between active layer 108and GaAs substrate 100 resulting in a stress ranging from 0.01% to 1%,and typically about 0.5% compressive stress for the embodiment shown inFIGS. 21 and 22. The amount and type of built-in active layer stress, ifany, depends on the particular alloy chosen for the quantum wells inactive layer 108. Possible alloys for quantum wells include InAlGaAs,AlGaInP, InGaAsN and AlGaAsSb. Second, reduction in the thickness ofAlGaAs layer 112 during the oxidation process also produces stress.Third, cavities may be used to induce stress.

The Hamiltonians for arbitrary wafer orientations may be obtained byperforming a unitary transformation on the (100) Hamiltonians:H′=U(θ,φ,γ) H U^(t)(θ,φ,γ), where U(θ,φ,γ) is the rotation operatorcorresponding to the Euler angles θ, φ, and γ of the substrate relativeto the (100) orientation. Once the Hamiltonians are determined, theenergy band structure may be solved for numerically. The gain curve as afunction of direction is then obtained by calculating the density ofstates and evaluating the relevant optical matrix elements.

The substrate orientation necessary to produce a desired gain versuspolarization angle dependency can be investigated, for example, by usingthe PICS3D software program available from Crosslight Software, Inc. at5450 Canotek Road, Unit 56, Gloucester, Ontario, Canada K1J9G4.

FIG. 26 shows VCSEL 400 with etched cavities 2601 and 2602 in accordancewith an embodiment of this invention. Aperture 422 is not asymmetric butcavities 2601 and 2602 are etched on either side of VCSEL 400, typicallyplaced as close as possible to aperture 422, to induce an asymmetry onactive region of VCSEL 400. One cavity or more than two cavities mayalso be used to generate differential loss and/or stress on VCSEL 400.Typically, cavities 2601 and 2602 are etched at the same time and usingthe same process as cavities 426. Hence, the depth of cavities 2601 and2602 is about the same as the depth of cavities 426. However, cavities2601 and 2602 can also be formed at a different time and using adifferent process from cavities 426. For example, cavities 2601 and 2602in an embodiment in accordance with the invention may be formed usingfocused ion beam milling subsequent to fabrication of VCSEL 400.Cavities 2601 and 2602 may be filled with a filler material having acoefficient of thermal expansion different from substrate 100 to enhancethe function of cavities 2601 and 2602 (see FIG. 3). For example,cavities 2601 and 2602 may be filled with a metal, semiconductor ordielectric material. The filler material is deposited at temperatureswell in excess of the operating temperature of VCSEL 400 so that as thefiller cools a stress is induced in VCSEL 400.

If VCSEL 400 is grown on misoriented substrate 2300, cavities 2601 and2602 may be oriented perpendicular to the direction of polarizationreinforced by misoriented substrate 2300 to further suppress thepolarization instability for VCSEL 400.

FIG. 27 shows VCSEL 500 with etched cavities 2701 and 2702 in accordancewith an embodiment of this invention. Aperture 522 is asymmetric andcavities 2701 and 2702 are etched on either side of VCSEL 500, typicallyplaced as close as possible to aperture 522, to reinforce the asymmetryon the active region of VCSEL 500. One cavity or more than two cavitiesmay also be used to generate differential loss and/or stress on VCSEL500. Typically, cavities 2701 and 2702 are etched at the same time andusing the same process as cavities 426. Hence, the depth of cavities2701 and 2702 is about the same as the depth of cavities 426. However,cavities 2701 and 2702 can also be formed at a different time and usinga different process from cavities 426. For example, cavities 2701 and2702 in accordance with an embodiment the invention may be formed usingfocused ion beam milling subsequent to fabrication of VCSEL 500.Cavities 2701 and 2702 may be filled with a filler material having acoefficient of thermal expansion different from substrate 100 to enhancethe function of cavities 2701 and 2702 (see FIG. 3). For example,cavities 2701 and 2702 may be filled with a metal, semiconductor ordielectric material. The filler material is deposited at temperatureswell in excess of the operating temperature of VCSEL 500 so that as thefiller cools a stress is induced in VCSEL 500.

If VCSEL 500 is grown on misoriented substrate 2300, cavities 2701 and2702 and the major axis of aperture 522 may be oriented perpendicular tothe direction of polarization reinforced by misoriented substrate 2300to further suppress the polarization instability for VCSEL 500.

While the invention has been described in conjunction with specificembodiments, it is evident to those skilled in the art that manyalternatives, modifications, and variations will be apparent in light ofthe foregoing description. Accordingly, the invention is intended toembrace all such alternatives, modifications, and variations that fallwithin the spirit and scope of the appended claims.

1. A method for making a vertical-cavity surface emitting laser devicefor producing polarized light comprising the steps of: providing asubstrate; forming a plurality of semiconductor layers on a surface ofsaid substrate wherein one of said semiconductor layers comprises anactive layer and another of said semiconductor layers comprises acurrent controlling layer having a first axis substantially orthogonalto a second axis in a plane substantially parallel to said surface ofsaid substrate, said current controlling layer being penetrated by aplurality of non-conducting cavities; forming an asymmetric apertureregion in said current controlling layer which controls current flowingthrough said active region, said asymmetric aperture region beingdefined by a conductive region in said current controlling layerbordered by non-conductive regions in said current controlling layersuch that a first dimension of said conductive region along said firstaxis is less than a second dimension of said conductive region alongsaid second axis to produce light polarized in a preferred direction,and wherein each of said non-conductive regions surrounds at least oneof said plurality of non-conducting cavities; and forming first andsecond electrodes located on said laser device to enable biasing of saidactive region.
 2. The method of claim 1 wherein said surface of saidsubstrate is misoriented with respect to the {100} and {111} planes ofsaid substrate resulting in polarization directions of maximum gain andwherein said directions of maximum gain are substantially aligned withsaid preferred direction.
 3. The method of claim 1 wherein a cavityhaving a width and a length is positioned adjacent to said asymmetricaperture region such that said length of said cavity is substantiallyparallel to said second dimension of said conductive region.
 4. Themethod of claim 3 wherein said surface of said substrate is misorientedwith respect to the {100} and {111} planes of said substrate resultingin polarization directions of maximum gain and wherein said directionsof maximum gain are substantially aligned with said preferred direction.5. The method of claim 1 wherein said first dimension of said conductiveregion is between 30% and 75% of said second dimension of saidconductive region.
 6. The method of claim 1 wherein said first electrodeis comprised of ITO.
 7. The method of claim 1 wherein said device is oneof an array of substantially identical devices emitting light havingsubstantially the same direction of polarization.
 8. The method of claim2 wherein said device is one of an array of substantially identicaldevices emitting light having substantially the same direction ofpolarization.
 9. The method of claim 3 wherein said device is one of aplurality of devices arranged in an array such that at least one of saidplurality of devices has a direction of polarization different from saiddevice.
 10. The method of claim 8 wherein said first electrode iscomprised of ITO.
 11. The method of claim 4 wherein said substratemisorientation is at least 4.1°.
 12. The method of claim 1 wherein saidcavities are located at the vertices of a distorted octagon.
 13. Amethod for making a vertical-cavity surface emitting laser device forproducing polarized light comprising the steps of: providing asubstrate; forming a plurality of semiconductor layers on a surface ofsaid substrate wherein one of said semiconductor layers comprises anactive layer and another of said semiconductor layers comprises acurrent controlling layer said current controlling layer beingpenetrated by a plurality of non-conducting cavities; forming anaperture region in said current controlling layer which controls currentflowing through said active region, said aperture region being definedby a conductive region in said current controlling layer bordered bynon-conductive regions in said current controlling layer and a cavityadjacent to said aperture region to induce an asymmetry on said activeregion so as to produce light polarized in a preferred direction, andwherein each of said non-conductive regions surrounds at least one ofsaid plurality of non-conducting cavities; and forming first and secondelectrodes located on said laser device to enable biasing of said activeregion.
 14. The method of claim 13 wherein said surface of saidsubstrate is misoriented with respect to the {100} and {111} planes ofsaid substrate resulting in directions of maximum gain and wherein saiddirections of maximum gain are substantially aligned with said preferreddirection.
 15. The method of claim 13 wherein said cavity is filled witha filler material having a first thermal coefficient of expansiondifferent from a second thermal coefficient of expansion of saidsubstrate.